[COLUG] Lisp variable scope in function

[Steven Huwig]

On Nov 30, 2006, at 4:51 PM, David Riggs wrote:

> Can anyone help me with my emacs lisp confusion?
>
> (defun mytest ()  (interactive)
>  (let ((dot '("a" .  0)) (n 0)) ;;set both to zero
> (setcdr dot (1+ (cdr dot)))  ;; incr both by one
> (setq n (1+ n))
> (print  dot)(print n)))

I think your trouble is using '("a" . 0) instead of (cons "a" 0).
Does this work?

(defun mytest ()  (interactive)
   (let ((dot (cons "a" 0)) (n 0)) ;;set both to zero
     (setcdr dot (1+ (cdr dot)))  ;; incr both by one
     (setq n (1+ n))
     (print  dot)(print n)))

This issue -- modifying a quoted literal list -- is mentioned
at the top of http://wiki.alu.org/Lisp_Gotchas .

> I have a little function to search through some data and count up
> occurances of found items, which I keep track of with an a-list.

Would this be something like:

(defun count-items (items)
   (let ((counts (make-hash-table)))
     (dolist (item items)
       (puthash item (1+ (gethash item counts 0)) counts))
     (let ((results '()))
       (maphash (lambda (k v) (push (cons k v) results)) counts)
       results)))

(count-items '(a b a a b c d b d))
((d . 2) (c . 1) (b . 3) (a . 3))

-- Steve